store: Support 'ssh://' URIs in 'GUIX_DAEMON_SOCKET'.

This allows 'guix' commands to talk to a remote store over SSH.

* guix/store.scm (connect-to-daemon)[connect]: Call 'resolve-interface'
for unknown URI schemes.
* guix/store/ssh.scm: New file.
* Makefile.am (MODULES): Add it.
* doc/guix.texi (The Store): Document it.  Mark remote access as
experimental.

Makefile.am

@@ -187,7 +187,8 @@ if HAVE_GUILE_SSH
 
 MODULES +=					\
   guix/ssh.scm					\
-  guix/scripts/copy.scm
+  guix/scripts/copy.scm				\
+  guix/store/ssh.scm
 
 endif HAVE_GUILE_SSH
 

doc/guix.texi

@@ -3696,9 +3696,30 @@ guix://master.guix.example.org:1234
 This setup is suitable on local networks, such as clusters, where only
 trusted nodes may connect to the build daemon at
 @code{master.guix.example.org}.
+
+@item ssh
+@cindex SSH access to build daemons
+These URIs allow you to connect to a remote daemon over
+SSH@footnote{This feature requires Guile-SSH (@pxref{Requirements}).}.
+A typical URL might look like this:
+
+@example
+ssh://charlie@@guix.example.org:22
+@end example
+
+As for @command{guix copy}, the usual OpenSSH client configuration files
+are honored (@pxref{Invoking guix copy}).
 @end table
 
 Additional URI schemes may be supported in the future.
+
+@c XXX: Remove this note when the protocol incurs fewer round trips
+@c and when (guix derivations) no longer relies on file system access.
+@quotation Note
+The ability to connect to remote build daemons is considered
+experimental as of @value{VERSION}.  Please get in touch with us to
+share any problems or suggestions you may have (@pxref{Contributing}).
+@end quotation
 @end defvr
 
 @deffn {Scheme Procedure} open-connection [@var{uri}] [#:reserve-space? #t]

guix/store.scm

@@ -411,6 +411,11 @@ (define addresses
 (define (connect-to-daemon uri)
   "Connect to the daemon at URI, a string that may be an actual URI or a file
 name."
+  (define (not-supported)
+    (raise (condition (&nix-connection-error
+                       (file uri)
+                       (errno ENOTSUP)))))
+
   (define connect
     (match (string->uri uri)
       (#f                                         ;URI is a file name
@@ -428,10 +433,21 @@ (define connect
                                  (errno EBADR))))) ;bah!
 
             (open-inet-socket (uri-host uri) (uri-port uri))))
+         ((? symbol? scheme)
+          ;; Try to dynamically load a module for SCHEME.
+          ;; XXX: Errors are swallowed.
+          (match (false-if-exception
+                  (resolve-interface `(guix store ,scheme)))
+            ((? module? module)
+             (match (false-if-exception
+                     (module-ref module 'connect-to-daemon))
+               ((? procedure? connect)
+                (lambda (_)
+                  (connect uri)))
+               (x (not-supported))))
+            (#f (not-supported))))
          (x
-          (raise (condition (&nix-connection-error
-                             (file (uri->string uri))
-                             (errno ENOTSUP)))))))))
+          (not-supported))))))
 
   (connect uri))
 

guix/store/ssh.scm

@@ -0,0 +1,39 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2017 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix store ssh)
+  #:use-module (guix ssh)
+  #:use-module (web uri)
+  #:export (connect-to-daemon))
+
+;;; Commentary:
+;;;
+;;; This modules provides the entry point for 'open-connection' in (guix
+;;; store).  Passing an 'ssh://' URI to 'open-connection' triggers the use of
+;;; the code in this module.
+;;;
+;;; End:
+
+(define (connect-to-daemon uri)
+  "Connect to the SSH daemon at URI, a URI object with the 'ssh' scheme."
+  (remote-daemon-channel
+   (open-ssh-session (uri-host uri)
+                     #:port (or (uri-port uri) 22)
+                     #:user (uri-userinfo uri))))
+
+;;; ssh.scm ends here